Concurrent schedules of reinforcement as a method to quantify the relative attractiveness of two rooting materials

Operant conditioning techniques were applied to investigate pigs’ preference for peat and long straw using concurrent schedules of reinforcement. Ten pigs worked, by pressing one of two panels, for access to two different materials in daily 50 min test sessions. Workloads of the reference material and the test materials, respectively, were varied in random order over 5 days with combinations of fixed ratios (FR) of 8/40, 16/32, 24/24, 32/16, 40/8. The slopes, intercepts and cross points of curves describing the number of rewards obtained of the reference material (peat) and the two test materials (peat and long straw) as a function of increasing workload of the reference material were estimated. Finally, the pigs worked for access to only one of the two materials at a time and the relation between number of rewards as a function of work load was estimated for the two materials. The increase in number of rewards of the test material with increasing workload for the reference material was higher for peat than for long straw (t301 = −2.4, P = 0.01) and the cross point for peat/peat was slightly lower (FR 26.7; 95% confidence interval: FR 23.8 to FR 29.6) than the cross point of long straw/peat (FR 31.0; 95% confidence interval: FR 27.5 to FR 35.0), indicating a slight preference for peat over long straw. There was no difference between the reference curves. Thus long straw substituted for peat. The validity of comparing the material on the reference panel with materials presented on the test panel was questioned.